The present invention relates generally to refrigerants for use in refrigeration systems, and more specifically to mixtures of carbon dioxide refrigerant and liquid cofluid for use in a CO2-cofluid refrigeration cycle with wet compression. More particularly, the invention relates to a method of selecting liquid cofluids which optimize the performance of such a refrigeration cycle.
Refrigeration systems are prevalent in our everyday life. They can be found in such varied locations as automobiles, refrigerators and freezers, air conditioning systems, supermarket display cases and many other applications. The most widely used refrigeration cycle in refrigeration systems is the vapor compression refrigeration cycle. In this cycle, the refrigerant is alternately compressed to condense the refrigerant, and decompressed to evaporate the refrigerant, thereby causing the refrigerant to transfer heat to and from the surrounding environment.
In the past, chlorofluorocarbons such as CFC-12 were the most commonly used vapor compression refrigerants. However, the phase out of chlorofluorocarbons, to protect the ozone layer, has caused a shift away from chlorofluorocarbons toward hydrofluorocarbon refrigerants, such as R-134a, a substitute refrigerant with no ozone depletion potential. More recently, concerns have arisen regarding the potential contribution of man-made refrigerant gases to global warming. Therefore, the search continues for refrigerants that are environmentally friendly.
Carbon dioxide is receiving increased attention as an alternative to today""s commonly used refrigerants. Carbon dioxide is attractive as a refrigerant because it is a xe2x80x9cnaturalxe2x80x9d material, present in abundance in the environment. The quantities used in refrigeration systems are too small to contribute to global warming. Consequently, significant attention has been directed to the use of carbon dioxide as a refrigerant using a transcritical carbon dioxide cycle. In this refrigeration cycle, the condenser of the traditional vapor compression cycle is replaced with an ultra-high pressure gas cooler and phase change does not occur.
A practical concern with the transcritical carbon dioxide cycle is its extremely high operating pressures. The components of the refrigeration system must be redesigned to withstand ultra-high pressures. Furthermore, the question of leakage control at ultra-high pressures has not been resolved. Another practical concern is efficiency, measured as the coefficient of performance (COP). The environmental benefit of an alternative refrigeration system is a function not only of the environmental friendliness of the refrigerant but also of its impact on the energy required (and consequently carbon dioxide produced) to satisfy the refrigeration requirements.
In a recent development, the environmental friendliness of carbon dioxide as a refrigerant has been retained while addressing its high operating pressure and limited efficiency by introducing a liquid cofluid into which carbon dioxide can absorb. The mixture of carbon dioxide and cofluid is used in a CO2-cofluid refrigeration cycle with wet compression. With a suitable choice of cofluid, absorption and desorption of carbon dioxide from solution replaces condensation and evaporation of pure carbon dioxide in the refrigeration cycle. These processes occur at significantly lower pressures while retaining adequate refrigeration capacity. However, currently known cofluids are not always optimal in terms of performance, environmental impact and/or commercial availability. Therefore, a need exists for additional types of cofluids having desirable properties for use in admixture with carbon dioxide in such a refrigeration cycle.
The present invention satisfies the above-described need by providing a carbon dioxide/cofluid mixture for use in a CO2-cofluid refrigeration cycle with wet compression. In this refrigeration cycle, the carbon dioxide is alternately absorbed and desorbed from the cofluid. The mixture comprises, by weight, from about 50% to about 95% cofluid and from about 5% to about 50% carbon dioxide. The cofluid is selected so that the mixture falls within region A of the plot of pvap versus xcex94hsoln shown in FIG. 3. The xcex94hsoln of the mixture is the differential heat of solution of the carbon dioxide in the cofluid at 5 wt % and 0xc2x0 C. The pvap of the mixture is the vapor pressure of the carbon dioxide over the solution at 20 wt % and 40xc2x0 C.
In another embodiment of the invention, the cofluid of the carbon dioxide/cofluid mixture has a thermal conductivity of greater than 0.12 W/m-K at 27xc2x0 C.
In another embodiment of the invention, the cofluid is selected so that the mixture falls within region A of the plot shown in FIG. 3, excluding cofluids 3, 11, 13, 15, 26 and 31 listed in Table 1.
In a further embodiment of the invention, the cofluid is selected from the group consisting of cofluids 4-9, 16-25, 27, 30, and 32-48 listed in Table 1.
The invention also relates to a method of screening a cofluid for producing the carbon dioxide/cofluid mixture. The method comprises the steps of: (a) determining through measurement or appropriate models pvap and xcex94hsoln for a carbon dioxide/cofluid mixture containing the cofluid to be screened; and (b) selecting the cofluid if it falls within region A of the plot of pvap versus xcex94hsoln shown in FIG. 3, and rejecting the cofluid if it does not fall within region A.
Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiments, when read in light of the accompanying drawings.